Audio reproduction systems generally consist of three parts. The first part is an audio source such as a phono turntable, a tape player, a disc player or an RF tuner for retrieving audio information and converting it to an electrical signal. The second part of the audio system is the loudspeaker. Loudspeakers serve to convert these electrical signals to acoustic waves to enable them to be perceived by a listener. Loudspeakers typically require a considerable amount of electrical power to create the sound pressure levels necessary for enjoyable listening- The third part of the audio system is a power amplifier. The power amplifier amplifies the electrical signals from the audio source and supplies the current necessary to drive the loudspeakers.
The heart of a modern power amplifier is an amplifier or series of amplifiers for supplying the current necessary to drive the loudspeakers. The power amplifier may consist of a transformer, vacuum tubes, BJT power transistors, MOSFET power transistors or other similar devices. Problems occur if the power capacity of the power amplifier is exceeded. If the power capacity is exceeded, catastrophic failure of the power amplifier can occur. Exceeding the power capacity of the power amplifier may also produce catastrophic failure of the loudspeakers.
Those skilled in the art of audio system design are aware of several methods of protecting the audio amplifier circuitry. One method is by the use of a traditional fuse or circuit breaker device. If the current flowing through the power transistors exceeds a certain pre-set amount, the current either blows the fuse or trips the circuit breaker providing an open circuit, thereby reducing the current to zero. One disadvantage of these systems is that they are relatively slow to respond. In some cases, the damage to the power amplifier has already occurred by the time the fuse or circuit breaker opens the circuit.
Other power amplifier systems such as U.S. Pat. No. 4,355,341 to Kaplan and U.S. Pat. No. 4,057,767 to Laupman utilize solid state circuitry on the output of the power amplification stage to sense the output current and either deactivate the input to the power amplification stage or deactivate the power amplification stage itself. The disadvantages of these systems are that the act of sensing the output current, by definition, distorts the output signal supplied to the loudspeakers. Any distortion of the output is undesired.
Alternate approaches are disclosed by U.S. Pat. No. 3,898,532 to Frank, U.S. Pat. No. 4,296,278 to Cullison et al., and U.S. Pat. No. 4,486,720 to Hirano which present methods whereby the audio amplifier circuitry is protected by either disconnecting the load from the power amplifier or by inserting a resistor in series with a load thereby reducing the power to the load. These systems also suffer the disadvantage of sensing the output current and thereby perturbing the output signal to the loudspeakers- Further, these systems also risk damage to the audio amplifier circuitry due to the manner in which they alter the load to the circuitry. Certain power transistor configurations can be adversely affected operating in a no-load condition. Other configurations are sensitive to drastic transitions between load and no-load, or load and reduced load conditions.